The present invention relates generally to ultra-high-speed fiber-optical communication systems and more particularly to optical time-division multiplexing (OTDM) systems and methods that provide improved performance and/or economy over conventional dense wavelength division multiplexing (DWDM) and other OTDM systems.
Modem computing and data communication applications are making ever-increasing demands on the communication systems to handle higher data rates per communication channel, as well as bandwidth. The increased demands are due not only to increased number of users and applications, but also to the increasing complexity of the applications. For example, large-scale distributed computing projects may involve multiple networked supercomputers exchanging data with other over the network at ultra-high speeds. Other examples include graphics-intensive network applications such as complex 3-D designer projects and interactive video applications involving multiple users located at large distances from each other. Another example is the use of storage area networks, in which large quantities of data are exchanged between users throughout a wide geographical area and large databases. Such networks may be used, for example, by commercial package delivery services that constantly track the delivery status nationwide or worldwide, credit card processing centers, large chain retail networks, etc. Many of these applications require network data transmission rates of 100 gigabits per second (Gb/s) or higher.
Optical fibers have been widely used as high-speed data paths. In a typical fiber optical communication system, optical signals are transmitted within ranges of wavelengths, or wavelength windows, that avoid unacceptably high optical absorption. For example, the International Telecommunications Union (xe2x80x9cITUxe2x80x9d) specifies six spectral bands for fiber optical communications: the O-Band (1,260 nm to 1,310 nm), the E-Band (1,360 nm to 1,460 nm), the S-Band (1,460 nm to 1,530 nm), the C-Band (1,530 nm to 1,565 nm), the L-Band (1,565 nm to 1,625 nm), and the U-Band (1,625 nm to 1,675). The C-band, for example, has range of useable wavelengths corresponding to a bandwidth of about 4.4 terahertz (THz) centered around wavelength of 1550 nm.
To maximize the utility of the signal-carrying capacity of the optical fiber, it is desirable to transmit signals having a combined wavelength content that occupies as much of the wavelength window as possible. FIG. 1 schematically shows a basic transmitter DWDM scheme for achieving this goal. Data from each data channel (one of Data_l through Data_N), delivered as electrical pulses, are used to modulate in electro-optical modulators (Ml through MN) emission of N different continuous-wave (CW) lasers with wavelengths from xcexl, to xcexN. The modulated signals are then multiplexed by a wavelength multiplexer (MUX) to produce a combined signal.
FIG. 2 schematically shows the spectrum (intensity I as a function of wavelength xcex) of the combined signal. Here, each data channel occupies a wavelength band centered at the wavelength permitted by the ITU and has a width that is proportional to the modulation rate of that channel. The WDM channels (xcex""s) must be sufficiently far apart to avoid crosstalk, or aliasing, between channels. Thus, there is some wasted wavelength space when wavelength multiplexing is used. Given the limited communication wavelength window (e.g., C-band, 35 nm), the higher the modulation rate of each channel, the fewer channels (with each channel containing more information) and the smaller total wasted space in guard bands between the channels.
Modulators in fiber-optical systems can be electronic or optical. In an electronic modulator, incoming optical data signals must be first converted to electrical signals. The electrical modulating signals are then sent to an electro-optic device to modulate CW lasers. The current modulation rate limit for electronic modulation is relatively low (e.g., Commercially available transmitters have a 10 Gb/s limit, with 40 Gb/s transmitters currently in prototype phase).
In direct optical modulation, non-linear optical materials are used. In a non-linear optical material, the refractive index of the material changes significantly with the intensity of the light passing through the material. When a high-intensity clock pulse propagates through such a material, the refractive index changes locally and affects the optical path of the data pulses, thereby modulating the data pulse. For example, in a modulator employing the principles of the well-known Mach-Zehnder interferometer, the data stream is split into two branches, at least one of which passes through a non-linear optical material. Clock pulses can be used to alter the refractive index of the material, thereby changing the phase of the data pulse in this branch relative to the other. The two branches are then recombined and will constructively or destructively interfere with each other, depending on the relative phase between the two, thereby forming modulated signals.
Optical modulators offer the possibility of much higher modulation rates than is possible with electro-optic modulators. Achieving higher bit rates can reduce or eliminate the need for expensive wavelength stabilized laser transmitters and DWDM multiplexers. Although they have been demonstrated in laboratory experiments, optical nonlinear modulators have so far not been shown to be cost effective solutions. As a result, existing fiber-optical communication systems that employ optical modulators in each separate channel tend to only be long-haul systems, where the expenses are more easily justified. There is, however, a long-felt need to reduce costs for all-optical systems in order to provide a cost-effective option for short-haul applications as well.
This invention is directed at solving one or more of the afore-mentioned problems.
Generally, the invention provides a device, system and method for economical implementation of OTDM systems. Instead of using a modulator for each data stream as in conventional systems, a device constructed in accordance with the principles of the invention employs an optical modulator to gate one or more data streams. A single stream of clock pulses, without splitting, is used to successively gate the one or more data streams. Thus, devices constructed in accordance with the invention are capable of achieving the same or better performance levels as conventional OTDM systems with fewer components. Because the clock pulses are not split to feed multiple modulators, relatively inexpensive, off-the-shelf, low power sources can be used.
According to one aspect of the invention, an optical device for serializing data signals in a plurality of channels includes: (a) one or more waveguides adapted to conduct light signals of a predetermined wavelength; and (b) a nonlinear optical element having a refractive index and defining an optical path therein adapted and configured to conduct a control light pulse along the optical path, wherein a portion of each of the plurality of waveguides is adjacent to or in contact with the nonlinear optical element at a different portion along the optical path; wherein the refractive index along the optical path is substantially altered where the control pulse is located such that the relative phase of the light signals of the predetermined wavelength is altered only where the signal is substantially coincident with the control pulse.
The refractive index along the optical path in the nonlinear optical element can be such that the light signals of the predetermined frequency in any one of the waveguides substantially cannot propagate past the point where the waveguide is adjacent to or in contact with the nonlinear optical element when the control pulse is not adjacent the point, and can substantially freely propagate through the point when the control pulse is adjacent the point.
The device constructed according to the invention can further include a single source for the control pulse, wherein the light path is adapted and configured to conduct the control pulse to all portions of the light path where the waveguides are adjacent to or in contact with the nonlinear optical element, whereby a single control pulse can alter the relative phases of all light signals substantially coincident with the control pulse propagating along the optical path.
The device of the invention can also further include an optical combiner adapted to spatially combine a plurality of optical beams into one beam, the combiner having a plurality of input ports operatively connected to the waveguides and an output port.
According to the invention, a method of serializing optical signals includes: (a) providing a plurality of optical signals in their corresponding waveguides; and (b) using a single control pulse without splitting the pulse, modulate the optical signals. The modulating step can include successively modulating the plurality of the optical signals. The successive modulation can be accomplished by propagating the control pulse in a non-linear optical media sequentially to points. The control pulse can be of same rate as the data pulse rate or can be an integer times of the data pulse rate for over-sampling.
The timing of the control pulses can be controlled such that they gate through the data pulses time wise substantially at the mid-point of the data pulses.